The present invention relates generally to the monitoring of output power of semiconductor laser diodes and particularly to the monitoring of the output power of laser diodes by detecting radiation emitted from the laser diode back facet.
It is well known in the scanning art to use diode lasers to generate a coherent laser beam which is used to scan a recording medium surface. It is also known to use multiple laser diodes to create multiple beams, each individual beam independently modulated by video signals, and the multiple beams scanned onto the recording surface as modulated beams. For these multiple beam applications, it has been found advantageous to use arrays of closely spaced laser diodes. Closely spaced diodes allow for multiple beam processing and thus improve data throughout as compared with older systems that employ continuous wave, single beam gas or semi-conductor lasers.
Typically, the laser diodes are individually addressable. Individual addressability generally requires that each diode have a separate current source that drives or modulates the diode. In operation, each driver sends a current through the diode sufficient to induce emission of laser light. The amount of current the driver produces is determined, in part, by the digital input data driving that particular lasing element. An example of a ROS system using a dual laser diode is disclosed in U.S. Pat. No. 4,796,964, whose contents are hereby incorporated by reference.
Because different laser diodes have different output power characteristics in response to a given driving current, it is desirable to monitor the amount of output power from each laser diode. If it is found that a particular diode is outputting too much or too little power at a given current level then the current needs to be adjusted to correct for the power differential. Laser diodes are typically constructed layer by layer from epitaxial deposition of appropriately doped semiconductor material. The front and back facets are then cleaved to produce reflective surfaces that define the front and back boundaries of the laser cavity. The front facet is designed to be more transmissive than the back facet which is generally made to be highly reflective. The front facet is thus the side from which the majority of laser light is emitted.
As stated above, the back facet is frequently also designed to be a highly reflective surface. However, some light ultimately escapes through the back facet of the diode. The amount of light leakage through the back facet is generally known to be proportional to the amount of light emitted from the front facet. This relationship between radiation from the back facet and the radiation from the front facet affords the opportunity to monitor the amount of output power from the front facet by detecting light emitted from the back facet.
To measure the amount of light from the back facet of a diode, a detector is typically disposed opposite the back facet of a single laser diode. In the case of a single laser diode configuration, one back facet detector gives complete information concerning the amount of radiation emanating from the front facet of that diode. In a multi-diode configuration, the confluence of concurrent, multiple beams does not give information concerning any particular diode.
FIG. 1 shows a top perspective view of a prior art Raster Output Scan (ROS) system 12 which includes a single laser diode 15 whose output is monitored by back facet detection. The ROS scans a data modulated beam 13 onto a xerographic photoreceptor drum 14 in accordance with a predetermined raster scanning pattern. ROS 12 comprises a laser diode 15 which is driven in accordance with image signals entered into, and processed by, ESS 16. Laser 15 emits light beam 13. A polygon scanner 17 is optically aligned between laser 15 and the drum 14 and rotated so that facets 18 intercept the output beams and cause the beams to be swept across the drum surface in a fast scan direction. Pre-scan optics 20 and post-scan optics 22 contain conventional optical elements which are used for beam forming and correction purposes.
The laser diode 15 has front and back facets 15A, 15B, respectively. While the majority of the laser light escapes from the front facet as beam 13, some radiation in the form of beam 13' is emitted from the back facet of the diode. This radiation is detected by a photosensor 24 which generates an output signal which is sent to ESS 16. ESS 16 then processes this signal comparing it to a predetermined voltage level corresponding to the desired power output of the diode. If correction is needed, a signal is sent to the drive circuit for the laser to increase or reduce the laser power output. As can be seen from FIG. 1, the light from the front and back facets spreads out in a conic shape. Other prior art disclosures which utilize a back facet detection are found in U.S. Pat. Nos. 4,342,050, 4,727,382 and 5,311,216.
For a multiple diode configuration, a single back facet photosensor opposite the laser diodes cannot simultaneously provide discernible information concerning the output power of any one laser diode since the overlapping of concurrent multiple beams does not give information concerning any particular diode. While each laser diode can be monitored separately by alternately turning each diode on and off, it is more efficient to be able to separately and simultaneously measure the light intensity of each laser diode.
Thus there is a need to construct an array architecture such that the amount of light emitted from individual back facets, of a multiple diode configuration is detected. Additionally, there is a need to regulate the output of the individual diodes in a continuous closed loop configuration to insure high print quality.
It is thus a first object of the present invention to provide a back facet monitoring system such that the amount of output power from individual back facts of laser diodes can be individually monitored in a continuous fashion.
There are additional prior art problems in back facet monitoring. In typical laser designs, up to 99.5% of the back facet is coated with a reflective material. Thus only a small amount of light (0.5%) is emitted from the back facet and is available to measure the power. Thus, photosensor 24 in FIG. 1, which is typically placed several millimeters behind laser 15, collects only a relatively small fraction of the already reduced light emitted from back facet 15B.
It is therefore a second object of the present invention to increase the sensitivity of the back facet light detector.
Another problem of prior art systems is stray light impinging on detector 24 distorting the output signal. The stray light is the result of reflections from the optical components in the system (e.g., from the optical components in pre-scan optics 20) being reflected from the rear facet and onto the detector. The detected signal will be distorted due to the optically induced "noise". For multiple diodes, increased "cross talk" results.
It is therefore a still further object of the invention to reduce the effects of stray light interference on back facet power monitoring detector signals.
These and other objects are realized by introducing an imaging component between the back facet and a small area photosensor with a fast response time. In one embodiment, the back facet light emissions from a dual emittor diode array are imaged onto 2 small photosensors formed on an array. More particularly, the present invention relates to an apparatus for monitoring the power output of at least a laser diode having at least a front and back facet, said apparatus comprising:
imaging means proximate said back facet and optically aligned with said back facet so that light emitted therefrom is imaged by said imaging means onto an imaging plane, and
at least one photosensor optically aligned with said imaging means and disposed in said imaging plane wherein said imaged light is focused onto said photosensor generating an output signal therefrom, said output signal being proportional to said power output.